Optical switches may be used in shutter applications, diaphragms, but also in switchable color filters in e.g. display applications. If the fluid is a (colored) oil and the second fluid is water (due to interfacial tensions), a two-layer system is provided which comprises a water layer and an oil layer. However, if a voltage is applied between the water and an electrode on the first support plate, the oil layer moves aside or breaks up due to electrostatic forces. Since parts of the water now penetrate the oil layer, the picture element becomes partly transparent.
Display devices based on this principle have been described in PCT-Application PCT/IB03/00196 (PH-NL 02.0129).
When used in a reflective display the optical performance of a picture element depends a. o. on the optical intensity of the colored film in the non-reflective state (off state) dyes (or sometimes pigments) are added to the oil (usually hydrocarbon but also possibly silicone or fluorocarbon), to provide sufficient optical intensity. Using the electro-wetting display principle the brightness in the reflective state (on state) is dependent on the scattering properties of the (underlying) substrate and the area fraction occupied by residual oil. This implies that the use of as little oil as possible will yield a high brightness. Contrast however also depends on the intensity and homogeneity of the optical intensity of the colored film in non-reflective state, so for high contrast a thicker layer of oil would be preferable.